ASK is a digital form of amplitude modulation (AM). The modulation used in this application is ASK, however, the terms AM and ASK are used interchangeably.
In high-frequency (HF) RFID/NFC systems signaling from interrogator/reader to tag or P2P, initiator to target, is affected by amplitude modulation of a carrier signal by the reader. In the case of ISO 14443B, FeliCa and ISO 15693, the AM index used is between 8% and 30%. The limits are tighter in some of the protocols. In the case of ISO 14443A with a data rate of 106 kbps, the amplitude modulation is 100% with up to 5% residuum signal allowed, which corresponds to so-called On-Off Keying (OOK) in effect.
The modulation index is an indicator of the level of modulation on an amplitude modulated signal. It is a measure of extent of the amplitude variation about an unmodulated carrier signal. The modulation index is defined according to RFID standards relevant for the present application, e.g. the NFCIP-1 standard, as the quotient of the difference between the peak and the minimum signal amplitude and the sum of the peak and the minimum signal amplitude. This is expressed in the following formula for the modulation index:
      (          a      -      b        )        (          a      +      b        )  
Therein a represents the peak amplitude and b represents the minimum amplitude of the modulated signal, respectively.
The modulation index is possibly expressed as a percentage. A modulation index of, for example, 8% can be expressed as a modulation depth of 14.8%.
Besides this basic difference in amplitude modulation index, the ISO specified protocols also differ greatly in the data signaling structure.
ISO 14443A and NFC forum NFC-A technology are based on OOK modulation using modified miller pulse position coding. FIG. 1 shows a modulated carrier signal with start of frame (SOF), data and end of frame (EOF), according to ISO 14443A. FIG. 2 depicts the corresponding envelope signal. The carrier itself cannot be discerned due to its high frequency. It can be seen that the SOF symbol heading the communication is composed of one pulse only. The EOF is marked by a non-modulated period.
ISO 14443B and NFC forum NFC-B technology employ direct bit coding. FIG. 3 depicts an accordingly modulated carrier signal and FIG. 4 the corresponding envelope signal. The SOF symbol is composed of ten to eleven low bits followed by two to three high bits. The EOF is represented by some low bits.
FeliCa protocol, also called NFC-F technology, is quite different. The data is Manchester coded on a sub-carrier clock of 212 KHz or 424 KHz. The SOF has a train of 48 or more unmodulated sub-carrier clocks. Beginning of data is marked by a first change in phase of the sub-carrier. EOF is signaled by the data. FIG. 5 shows the resultant modulated carrier signal and FIG. 6 the corresponding envelope signal.
Due to the use of OOK and ASK, two types of demodulators are typically employed: an OOK demodulator and an ASK demodulator. The ASK demodulator must operate on a span of allowed modulation indexes, e.g. 8-30% in case of FeliCa and 8-14% in case of ISO 14443B. In case of ISO 14443B, the demodulator must detect even the first amplitude change with sufficiently small timing distortion due to the structure of the SOF. On the other hand, the FeliCa signaling allows the demodulator to use the starting clock signals to perform settling of the demodulator. Therefore, the timing distortion on starting clocks can be higher than allowed but is reduced below the allowed level after a few clocks, e.g. ten. The time distortion or pulse width (PW) distortion introduced by the demodulator that can be tolerated by a subsequent digital signal processing is quite different for ISO 14443B and FeliCa. The timing distortion of the demodulator is mainly caused by the non-ideal, i.e. sloped AM signal shapes. Looking at the ratio of allowed slopes of AM signals as a percentage of bit duration in the case of ISO 14443B, the sloping can take up to two times 16 carrier cycles which amounts to approximately 3.6 μs out of 9.44 μs a bit duration which accounts for roughly 25% of bit duration. FeliCa 212 allows a maximum slope of 2 times 2 μs out of 4.8 μs bit duration which makes up approximately 80% of bit duration.
The requirement for ISO 14443B demodulation is to receive each, i.e. even the first AM modulation transition correctly and with low enough PW distortion. The requirement on PW distortion, however, is not as strict as for the FeliCa protocol.
The requirements for the FeliCa protocol are very strict on PW distortion, but the demodulator can use a few starting AM modulated clock pulses as a means to settle the demodulator. This settling is needed as the incoming envelope signal can differ greatly in the initial signal level, i.e. in the amplitude of the AM signal due to different AM modulation indexes used and in the sloping of AM envelope signal.
To meet those conflicting demands, two AM demodulators of different structure are usually employed in known solutions.
In a state of the art demodulator for ISO 14443B, the ASK modulated signal is rectified and filtered to generate the envelope signal. The demodulation operates on the principle of comparing the envelope signal with a reference signal, which is generated by low pass filtering the envelope signal. The time constant of the reference voltage generation is adjustable and can track the data rate. The comparison of the envelope and the reference signal is performed on a comparator which outputs the digitized demodulated signal. The output of the comparator also serves as the control for the comparator threshold voltage sign, which is added to the reference voltage to create a hysteresis. The threshold voltage magnitude is dependent on the offset of the comparator. The output of the comparator is connected to a noise remover circuit which rejects short pulses by delaying the output signal. Operation of a corresponding state of the art demodulator is shown in FIG. 7. Before the first modulation pulse, the reference signal settles to the same level as the envelope signal. The hysteresis is set such that the threshold voltage with hysteresis is below the envelope signal. When the comparator detects the crossing event, it inverts the hysteresis such that the threshold voltage is above the envelope signal. The reference voltage is filtered with a filter that has a predefined time constant. Unfortunately, the demodulator is unsuitable for ISO 14443A signals.
A state of the art demodulator for FeliCa typically uses AC-coupling of the signal to achieve the required settling of the demodulation. Such an AC-coupled ASK demodulator circuit is composed of an antenna, a rectifying diode acting as an envelope detector, a capacitor connected in series, an amplifier and a data slicer. The RF signal received by the antenna is first rectified by the diode and then passed through the capacitor. The capacitor cuts away the DC component and passes through only the AC component, acting as a differentiator for the envelope signal. The resulting differentiating signal is amplified and supplied to the data slicer, which uses a predetermined threshold and converts the analogue signal to data symbols “0” and “1”. Corresponding signals are depicted in FIG. 8. The data slicer has a window comparator with an upper and a lower reference. When the differentiated signal crosses the lower reference signal with a falling edge, the digitized signal changes its level to high. As soon as the differentiated signal crosses the upper reference signal with a rising edge, it changes the level of the digital signal to low. When the lower reference value is crossed with a rising edge and the upper reference signal with a falling edge the digitized signal level does not change.
As the use of two demodulators leads to an increase in cost of the implementation, there is a need in the art to define a single demodulator circuit and corresponding method which satisfy the conflicting requirements of the different standards used in HF RFID as described above.